1. Field of the Invention
The invention relates to a screened switching apparatus having, on the one hand, two contact pieces movable with respect to each other between a closed state in which the contact inserts are touching and an open state in which the inserts are separated and, on the other hand, an isolating screen which comes rapidly between the inserts by moving from a starting position to a work position, during which movement the arc appearing on opening is displaced by an active edge of the screen and at the end of which the arc is totally destabilized.
2. Description of the Prior Art
Electric switches are already known using arc deflector devices in the form of horns placed in the vicinity of the contact inserts at the free ends of the contact pieces and which have, with respect to the general direction of these pieces, a change of orientation such that the natural development of the arc formed under the effect of the electrodynamic repulsion forces brings the feet of these arcs onto these horns and consequently preventing them from remaining on said inserts.
The orientations, curvatures, shapes or positions given to these horns are essentially based on the assumption that the current which flows in the arc is sufficiently high for this development to be possible; the same measures may be taken if development or swelling of the arc is assisted by the presence of a local magnetic field which may be induced or permanent and which is suitably orientated to promote rapidity of the displacement.
In these known devices, the desired increase of the arc tension results from its extension, whereas the fact of installing the feet of these arcs on the horns avoids consumption of the noble metal of the contact inserts; in such known devices, the orientations of these horns are generally divergent.
When, in order to obtain limitation of the arc current the length of this latter is not greatly increased (for example, because it is not possible to give high amplitude movements to the mobile contact), when the arc current which it is desired to limit does not reach the thresholds from which the presence of the electrodynamic repulsion forces may be readily used, or when the conformation, respectively the construction of the quenching chamber or of the adjacent conductors prevent free expansion of the arc or respectively splitting up of the arc on fins, artificial means are used for destabilizing the arc; an isolating screen rapidly interposed between two contacts which have just opened forms one of them and has numerous advantages, provided that modifications are made to the characteristics of the arc which will be other than its length, and which will still have as result an increase of its tension.
Since important factors in destabilization of the arc require a high speed of the active edge of the screen and quenching of this arc against an isolating wall, in this case it is necessary to take into account the fact that the contact inserts have not the time to move apart by an appreciable distance when the screen moves between them, and that the arc is confined in the vicinity of the contacts by walls of the case.
The result is that, lacking special measures, the arc which tends then to follow a path of minimum length risks remaining unduly on these inserts and consequently causing deterioration thereof which adversely affects the quality of subsequent switching or closure.